This invention relates to the field of fiber optic communications and, more specifically, to monitoring the status of optical nodes. This invention also relates to hybrid fiber optic and coaxial cable networks.
Oftentimes, in common fiber optic networks, especially cable television systems, it is preferable to monitor the status of a fiber optic transmission node (or optical node) downstream from the transmitter in order to detect transmission errors. This is especially advantageous within hybrid fiber optic and coaxial cable (xe2x80x9cHFCxe2x80x9d) networks where optical nodes increase in volume closer to the residential neighborhood level to service relatively small numbers of homes passively with the coaxial cable portion of the network. By incorporating a monitoring system into a forward path broadband transmitter and the optical detection section of a broadband optical receiver, control signals can be sent to a remote optical receiver for more convenient monitoring.
The prior art discloses utilizing an RF transponder for monitoring the transmission health of a remote optical receiver. The monitoring signals from the RF transponder are sent downstream to an optical receiver along with a principal RF signal (carrying video, audio or data transmissions). While this configuration is useful for its intended purpose, the equipment necessary is often bulky and, therefore, physically constraining. RF transponders are power consuming relative to digital circuits and, because they utilize analog transmission techniques, are relatively unreliable. Further, the RF transponder monitoring system utilizes a downstream communications channel within the typical HFC spectrum that infringes upon the bandwidth that can be allocated for the main transmission signal. This bandwidth infringement adversely affects the efficiency of the network. The effect of such usage is the degradation of signal speed, quality and strength. Also, the complex synchronization of the monitoring signals within the downstream communications channel and the RF tuner and demodulator pairs necessary for synchronization further serve to make such systems complex and unreliable.
In addition to the RF monitoring signal, return laser dithering signals are also sent from the transmitter the nodes. A return laser dithering (periodically or randomly varying) signal is usually used in order to stabilize a lightly loaded laser and improve the left (linear) side of the noise power ratio (NPR) curve, however such a signal could be modified to carry intelligent information. In a typical ERDx return transmitter (single fiber for reverse transmission back to the hub or head end), laser diode dithering can be accomplished by the addition of pseudo-random noise below 5 MHz. Such a signal can be generated by a digital signal processor.
There is a need in the art for a more efficient way of monitoring the health of a remote optical receive site while eliminating the need for physically constraining, often unreliable and power consuming RF transponders.
There is also a need in the art for a method to allow all transmissions of status information to be performed outside of the bandwidth used for services on the HFC network. A monitoring system that could operate without infringing upon the bandwidth allocated for services would improve service transmission quality and efficiency.
There is a further need in the art for the elimination of the problem of complex synchronizing and collision detection that is required when RF transponders are used for transmitting status information.
Finally, there is a need in the art for a method to eliminate the use of a downstream communications channel within the HFC spectrum for the complex synchronization of monitoring signals. Such a method could increase reliability by eliminating the need for an RF tuner and demodulator.
The present invention overcomes the above-described problems in the prior art by providing a method for implementing an integrated status transponder using modulation of the return laser dithering signal. The present invention provides a method for transmitting an optical node monitoring signal corresponding to error detection parameters relating to the reception of a principal RF signal over an optical network extending from a transmission site to a receiver. A principal RF signal is received at the transmission site. A stabilizing RF signal and an optical node monitoring signal based on error detection parameters relating to a primary RF signal are generated at the transmission site. The two signals are modulated to generate an integrated signal. The integrated signal and the principal RF signal are sent in conjunction over an optical network to a receiving site. The integrated signal and principal RF signal are received from the optical network by a receiver for verifying the integrity of the transmitting node.
Objects, features and advantages of the present invention will become apparent upon reading the following detailed description of the preferred embodiments of the invention, when taken in conjunction with the accompanying drawings and appended claims.